This invention relates generally to techniques for measuring a decaying electromagnetic radiation signal, and, more specifically, to the measurement of a decay time of a luminescence signal that is generated in a material exposed to some parameter to be measured thereby. Measurement of temperature is extensively discussed herein but the invention is not limited to temperature applications.
One such application is the remote, real-time measurement of surface temperature distributions. In wind tunnel experiments with aircraft models, for example, a sequence of thermal images of the surface of the model aircraft may be acquired as part of the test data. Changes in the temperature distribution across the surface of an operating electronic component or complete circuit board is another application. In the medical field, medical thermography involves a thermal imaging of a skin area of a patient to provide information helpful to diagnosing the patient's condition. One way of doing this is to scan an infrared image of the surface across a point detector having an appropriate response to obtain electronic signals which are then used to display an image in visible light. The significant disadvantage of this technique is the resultant complexity inherent in the optomechanical and infrared detection technology used for the imaging system in order to achieve good speed, sensitivity and spatial resolution. Also, although direct detection of the infrared image can provide an acceptable qualitative visual representation of the temperature profile across the surface being viewed, the absolute accuracy of the measurement of temperature may be limited for a variety of reasons.
Another approach to observing and measuring the temperature across the surface is to first convert the temperature variations of the surface into thermally-encoded visible or near visible emissions before the radiation is detected and electronically processed. One such technique is to position a layer of luminescent material in thermal communication with the surface, such as by coating the luminescent material directly onto the surface. More conventional and less expensive imaging devices may then be used to detect and process the visible or near visible luminescent emissions. One such technique is to coat the surface with conventional thermographic phosphors and then detect the intensity of the luminescent image, much like any other optical image. This provides a good visual representation of temperature variations across the surface but suffers from a limited range of measurement and inaccuracies when quantitative measurements of temperature are desired. Thus, others have suggested variations of the luminescent material plus further optical processing of the luminescent image, such as by a pixel-by-pixel ratioing of the intensities of two separate and thermally dissimilar wavelength bands of luminescent emission. Given the right optics and luminescent material, this ratio is proportional to temperature. It has also been suggested to measure, on a pixel-by-pixel basis across the image, the decay time of the luminescence of a coating after the coating is excited by a pulse of excitation radiation. The decay time of the luminescent emission of selected materials is proportional to the temperature of the luminescent material over a given range of interest.
However, a totally satisfactory approach to such luminescent image decay time analysis does not yet exist. Therefore, it is a primary object of the present invention to provide this needed solution. An important goal of the present invention is a low-cost, simple, reliable and easy to use system that gives fast, accurate temperature measurements across a two dimensional luminescent surface over a wide range of temperatures.
Use of the luminescent decay time technique for measuring the temperature of a single small spot of luminescent material is becoming widespread. Optical fiber temperature measuring systems are commercially available. A very small quantity of luminescent material is formed as part of a sensor at the free end of an optical fiber, the other end of the optical fiber being connected to a measurement instrument. The instrument repetitively sends pulses of excitation radiation down the fiber and receives back from the sensor, in between the pulses, the decaying luminescent signal which is typically approximately exponential in time. A quantity proportional to the luminescent decay time, which is indicative of the temperature of the sensor, is then obtained by one of several signal processing techniques. One such technique is to measure the time it takes for the decaying intensity signal to fall from one value to another value that is the first value divided by e. This time is by definition the decay time of the luminescence. Another technique is to integrate the decaying luminescent signal over two different periods and then compare the integrated values, such as by ratioing them. Yet another technique is to digitize the decaying luminescent signal and subsequently analyze the digitized data to determine the decay time from the best fit of an exponential curve to the data samples.
Currently, a typical fiberoptic temperature measuring system includes only one or just a few optical fiber probes connected to a common optical instrument and signal processing system. It is another object of the present invention to provide such a instrument and system that can be used to multiplex hundreds or even thousands of separate optical fiber probes, and thus provide separate temperature measurements from each of the probes at a reduced cost per probe relative to what is now possible.